Polymer brushes are a broad class of materials comprised of polymer chains that are tethered by one chain end to a surface. These brushes have a variety of applications due to their ability to tune and modify surface properties such as bioadhesion, wettability, and surface activity.
Two main methods for the preparation of polymer brushes have emerged, namely grafting “to” and grafting “from”. The grafting “to” methodology involves the reaction of an end-functionalized polymer chain with an appropriate surface to anchor the polymer. Although grafting “to” allows for full characterization of the polymer before grafting, it is only applicable to a limited number of substrates and requires terminal functionality on the polymer chain-end. In addition, the grafting efficiency decreases with increasing molecular weight of the polymer.
Grafting “from” overcomes some of these limitations and has been used with a variety of polymerization techniques. By anchoring a suitable initiator to the substrate, polymer chains can be grown directly by the use of these various polymerization techniques. The majority of surface-anchored initiators comprise a self-assembled monolayer (SAM) on an appropriate substrate. However, SAMs have limited stability to various reagents and are not substrate-independent.
Surface-initiated atom transfer radical polymerization (SI-ATRP) has become a workhorse in the grafting “from” literature due to the ease in polymerizing a wide variety of monomers containing an array of functional groups with a high degree of control. Control in ATRP comes from the reversible redox activation of a dormant polymer chain-end (halide functionalized) by a halogen transfer to a transition metal complex. Many parameters are involved which can be tuned for better control, which provides an impressive window in which well-controlled polymers of numerous different monomers can be synthesized.
While the most common method for anchoring ATRP initiators to a substrate involves the formation of a SAM, some alternative methods have been presented in the literature. von Werne et al. describe the inclusion of 10˜20% ATRP inimer in a mixture of curable monomers suitable for photopolymerization. (See, von Werne, T. A.; Germack, D. S.; Hagberg, E. C.; Sheares, V. V.; Hawker, C. J.; Carter, K. R., A Versatile Method for Tuning the Chemistry and Size of Nanoscopic Features by Living Free Radical Polymerization. J. Am. Chem. Soc. 2003, 125, 3831-3838.) This work was further extended by the use of an acid-cleavable ATRP inimer, allowing for direct measurement of surface grown brushes and their comparison with polymer grown from sacrificial initiator in solution. (See, Koylu, D.; Carter, K. R., Stimuli-Responsive Surfaces Utilizing Cleavable Polymer Brush Layers. Macromolecules 2009, 42, 8655-8660.) An alternate method for creating an inimer layer is to form an adhesive coating which contains moieties for initiator incorporation. For example, layers of poly(allylamine) (deposited by pulsed plasma polymerization) or catechol-amine (deposited by solution incubation) on various substrates were used for functionalizing a surface with ATRP initiators. (See, Yameen, B.; Khan, H. U.; Knoll, W.; Förch, R.; Jonas, U., Surface Initiated Polymerization on Pulsed Plasma Deposited Polyallylamine: A Polymer Substrate-Independent Strategy to Soft Surfaces with Polymer Brushes. Macromol. Rapid Commun. 2011, 32, 1735-1740, Coad, B. R.; Lua, Y.; Meagher, L., A Substrate-Independent Method for Surface Grafting Polymer Layers by Atom Transfer Radical Polymerization: Reduction of Protein Adsorption. Acta Biomaterialia 2012, 8, 608-618, Fan, X.; Lin, L.; Dalsin, J. L.; Messersmith, P. B., Biomimetic Anchor for Surface-Initiated Polymerization from Metal Substrates. J. Am. Chem. Soc. 2005, 127, 15843-15847.) More recently, a catechol-functionalized methacrylamide and a methacrylate ATRP inimer were copolymerized by free radical polymerization followed by deposition on Ti substrates for polymer brush growth. (See, Wang, X.; Ye, Q.; Gao, T.; Liu, J.; Zhou, F., Self-Assembly of Catecholic Macroinitiator on Various Substrates and Surface-Initiated Polymerization. Langmuir 2012, 28, 2574-2581.)
Polymer brushes patterned with nanoscale features have been generated by patterning a polymer brush growth-initiating substrate using various techniques, followed by polymer brush growth from the patterned substrate. For example, microcontact printing has been used to form a pattern in a substrate, followed by backfilling portions of the patterned substrate with a polymer brush growth-initiating material. Nanoimprint lithograph has also been used to form a pattern in a polymer-brush growth-initiating substrate. Alternatively, ultraviolet (UV) or electron beam lithography has been used to form a pattern in a polymer brush growth-initiating substrate.